1. Field of the Invention
The present invention relates to a semiconductor device having trenches and a method for the same, in particular, to an improvement of the functions of a semiconductor device having trenches as well as to an improvement of the yield.
2. Description of the Background Art
FIGS. 18 and 19 are schematic cross section views showing a method of filling in a trench in accordance with the order of the steps according to a prior art. First, referring to FIG. 18, a trench 102 is formed in the surface of a semiconductor substrate 101 using, for example, anisotropic etching.
Referring to FIG. 19, an insulating film 103 made of, for example, a silicon oxide film is formed on the semiconductor substrate 101 so as to be filled into this trench 102 through, for example, a CVD (Chemical Vapor Deposition) method. In this way, the trench 102 is filled in.
In a method of filling in the trench 102 according to the prior art, however, the insulating film 103 has the characteristic that it is deposited more at the aperture part R1 of the trench 102 than inside of the trench 102. Therefore, in the case of a trench 102 which has a large aspect ratio (depth D/width W), the trench 102 is closed at the aperture part R1 before the inside of the trench 102 is completely filled in. Therefore, the trench 102 cannot be filled in completely and, in many cases, a void 102a is left within the trench 102. Especially, in the case that the aspect ratio of the trench 102 exceeds 10, or in the case that silicon oxide film (SiO2) which is formed through a CVD method is used for filling, this phenomenon becomes significant.
In the case that the void 102a which is left inside of the trench 102 is large, there is a possibility that it affects the electric characteristics (main breakdown voltage or the like) of the semiconductor device. In addition, there are cases where thermal expansion and shrinkage are repeated in the vicinity of the void 102a due to temperature change in the operational environment of the semiconductor device so that the reliability for a long period of operation time is significantly harmed. Particularly, in the case that there is a void 102a in the vicinity of a pn junction to which the largest electric field of the device is applied, the above described problem of the reliability becomes more significant. In addition, in many cases of a power semiconductor device which is exposed to a high temperature and, therefore, this problem is significant.
The film thickness of the filled in material 103, with which the trench 102 is filled in, essentially needs to be xc2xd, or more, of the width W of the trench 102 at the minimum. Therefore, the wider the width W of the trench 102 is the thicker the film thickness T, of the insulating film 103 which is deposited on the surface of the semiconductor substrate 101, is according to the width W of the trench 102. In the case that the film thickness T of the insulating film 103 which is deposited on the surface of the semiconductor substrate 101 is large, the step between respective surfaces of the semiconductor substrate 101 and the insulating film 103 becomes large. Therefore, a problem arises in the structure, or in the following manufacturing steps, wherein the formation of contact holes or the patterning of a conductive layer in the vicinity of this step become difficult. 
In order to solve the above described problem caused by the above step, the insulating film 103, which is deposited on the surface of the semiconductor substrate 101 after being filled in into the trench 102, may be removed. It is difficult, however, to remove only the insulating film 103 which is deposited thickly on the surface of the semiconductor substrate 101 while leaving most of the insulating film 103 inside of the trench 102 by removal through ordinary dry and wet etching methods. As for a comparatively excellent method, there is a method of removing the insulating film 103 which remains on the surface of the semiconductor substrate 101 by using a CMP (Chemical Mechanical Polishing) method, or the like. Even in the case when this method is used, however, the void 102a, which has been formed inside of the trench 102, cannot be eliminated.
The purpose of the present invention is to provide a semiconductor device and a process for the same which can reduce the void inside of the trench in a simple method and which can make thinner the film thickness of the insulating film for filling in the trench which remains on the surface of the semiconductor substrate.
In a semiconductor device having a trench according to the present invention, elements and trenches are alternately and repeatedly arranged in the main surface of the semiconductor substrate wherein each of the plurality of elements arranged alternately with trenches repeatedly has a structure which is operated in the same operation mode and the insulating film which is filled in into the trenches does not have any voids in the vicinity of the pn junction to which the largest electric field of the element is applied or in the positions shallower than the pn junction.
In the semiconductor device having the trench according to the present invention this void can be prevented from affecting the element since the insulating film which fills in into the trench does not have any voids in the vicinity of a pn junction to which the largest electric field of the element is applied or in the position shallower than the pn junction.
In the case that the aspect ratio of the trench is 10, or more, in the above described semiconductor device, the occurrence of the void within the trench can be prevented particularly effectively.
In the case that the depth of the trench is 5 xcexcm, or more, in the above described semiconductor device, the occurrence of the void within the trench is particularly prevented so that the harmful effect to the element by the occurrence of the void can be significantly prevented.
In the above described semiconductor device, in the case that the element has the first impurity region of the first conductive type which is formed on one side of the mesa region sandwiched between adjacent trenches, the second impurity region of the second conductive type which is formed on the other side of the mesa region and which forms a pn junction with the first impurity region and the third impurity region of the second conductive type which is formed on at least part of the main surface of the first impurity region, the harmful effect to the element due to the occurrence of the void can be particularly significantly prevented since the influence given to the element by the void within the trench is large.
In the above described semiconductor device, in the case that the third impurity region is a body part of an insulating gate-type field effect transistor and the pn junction to which the largest electric field of the element is applied is formed of the third impurity region and the first impurity region, the harmful effect to the element due to the occurrence of the void can be particularly significantly prevented since the electric field which is applied to the pn junction is large so that a large electric field is easily applied to the void.
A process for a semiconductor device according to the present invention includes the step of forming a trench in the main surface of a semiconductor substrate, the step of forming a first insulating film on the main surface of the semiconductor substrate in one step or in a plurality of divided steps so as to be filled in to the trench, the step of forming an aperture (opening portion) which reaches to the inside the trench in the first insulating film through an anisotropic etching in the first insulating film so that the upper edge corner parts of the aperture in the first insulating film has more round edge than the upper edge corner parts of the trench and of reducing the film thickness of the insulating film on the main surface of the semiconductor substrate and the step of forming a second insulating film on the main surface of the semiconductor substrate in one step or a plurality of divided steps so as to be filled in to the aperture.
According to the process for a semiconductor device of the present invention, the upper edge corner parts of the aperture are made to have round edge through an anisotropic etching. Therefore, the second insulating film can be prevented from closing the upper edge parts of the aperture before being filled into the aperture at the time of the formation of the second insulating film. Thereby, it becomes possible to reduce the void within the aperture or completely eliminate the void by filling in the aperture with the second insulating film so that the harmful effect to the element due to this void can be prevented.
In addition, since the film thickness of the insulating film on the main surface of the semiconductor substrate can be reduced through an anisotropic etching, the insulating film for filling in the trench can be prevented from remaining thickly on the main surface of the semiconductor substrate. Therefore, the disadvantage that the insulating film which has remained thickly causes a large step between the main surface of the semiconductor substrate and the upper surface of the insulating film can be improved.
In the above described process for semiconductor substrate, preferably the step of forming the first insulating film, the step of etching anisotropically the first insulating film and the step of forming the second insulating film are repeated twice, or more, in this order.
Thereby, the inside of the trench of which the aspect ratio is significantly large can be filled in with the insulating film without producing a void.
In the above described process for a semiconductor device, the first and the second insulating films are, preferably, any of a silicon oxide film, a silicon nitride film and a complex film of a silicon oxide film and a silicon nitride film, formed through a chemical vapor deposition method.
Thereby the first and the second insulating films can be easily formed, formed through a chemical vapor deposition method.
In the above described process for a semiconductor device, before the completion of the filling in of the aperture with the first insulating film or before the completion of the filling in of the aperture with the second insulating film, preferably sintering treatment is carried out in order that the insulating film within the trench is made higher in the density physically or stoichiometrically so as to reduce the stress and that the stress occurrence due to the thermal treatment in the following steps is suppressed.
Thereby, a breakdown of the semiconductor device caused by the difference in the heat expansion between silicon of the semiconductor substrate and a silicon oxide film of the insulating film can be prevented.
In the above described process for a semiconductor device, the sintering treatment is carried out at a temperature of 800xc2x0 C., or more, and under a water vapor atmosphere, a burning oxidization atmosphere or an atmosphere of a high partial pressure of oxygen (an atmosphere with high oxygen content).
Thereby, the stress of the silicon oxide film can be effectively reduced.
In the above described process for a semiconductor device, preferably, the temperature in the sintering treatment is higher than in any other temperatures in the following steps.
Thereby, the stress occurrence due to the thermal treatments in the following steps can be prevented.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.